Magnetoinductive flowmeters are well-known from prior art, described, for example, in “K. B. Bonfig, Technische Durchflussmessung (industrial flow measurements), 3rd edition, Vulkan-Verlag, Essen, 2002, pp. 123-167”. The underlying concept of a magnetoinductive flow-measuring device goes all the way back to Faraday who in 1832 proposed using the principle of electrodynamic induction for flow-rate measurements.
According to Faraday's law of induction, a flowing medium that contains charge carriers and travels through a magnetic field will produce an electric field intensity perpendicular to the direction of flow and perpendicular to the magnetic field. A magnetoinductive flowmeter utilizes Faraday's law of induction in that by means of a magnetic-field system containing at least one magnet with typically two field coils a magnetic field is generated and positioned over the cross-sectional area of the measuring tube, which magnetic field includes a magnetic-field component that extends in a direction perpendicular to the direction of flow. Within the magnetic field, each volume element of the flowing medium, containing a certain number of charge carriers, contributes the field intensity created in that volume element to the measuring voltage that can be collected via the electrodes.
One salient characteristic of magnetoinductive flowmeters is the proportionality between the measured voltage and the velocity of the flowing medium through the cross section of the measuring tube, i.e. between the measured voltage and the volume flow. Apart from the electrodes serving to collect the voltage being measured, additional electrodes may be provided, such as zero-flow detection electrodes as well as grounding electrodes especially also in the form of grounding sleeves.
Metal electrodes that are in direct contact with the flowing medium form an electrochemical boundary layer, capable of producing electrochemical direct-current voltages whose order of magnitude may be several 100 mV. These electrochemical direct-current voltages can change quite rapidly, for instance as a function of variations in the local flow rate of the medium at the electrodes due to turbulences, of operating-pressure fluctuations, of the pH value of the medium, of the composition of the medium especially while chemical reactions are still going on in the medium, of solid particles carried by or particles suspended in the medium and interfering with the boundary layer on the electrodes, or of solid particles in contact with or indeed impinging on the electrodes. All these factors lead to statistical fluctuations of the electrochemical direct current at the electrodes with amplitudes ranging from a few μV to several 10 mV. These statistical voltage fluctuations are also referred to as random noise.
This random-noise voltage is superimposed on the flow-proportional signal voltage whose signal intensity is typically 0.1 to 1 mV/(m/s), i.e. 0.1 to 1 mV for a flow velocity of one meter per second. It follows that the amplitude of the random-noise voltage may be of the same magnitude as the flow-proportional signal voltage or even well above that. The result may be substantial noise interference with consequently irregular, strongly fluctuating flow-rate readings of the magnetoinductive flowmeter.
That problem has been addressed in the prior art by applying on the surface areas of the electrodes that come in contact with the medium a porous coating consisting of a nonconducting, electrically inert material, for instance a porous ceramic layer. The electrically conductive medium penetrates into these pores, creating a voltaic connection between the medium and the metal of the electrodes and their associated transducer. This has a number of beneficial effects:
The medium that makes contact with the metal of the electrodes is replaced very slowly. This significantly minimizes sudden, measurement-disrupting spikes of the direct-current voltage at the electrodes as a result of an electrochemically heterogeneous medium. Moreover, solid particles carried by the medium are no longer able to penetrate into the electrochemical boundary layer that is now largely protected by the ceramic coating. Solid particles carried by the medium therefore cause significantly less interference than would be the case without that porous ceramic layer.
To be sure, this type of porous ceramic layer has drawbacks of its own. For example, abrasion or physical impact can damage the ceramic layer whenever the medium carries along hard solids. Then, too, the chemical resistance of the ceramic material used is not always adequate. The same is true for the thermal-shock resistance of these ceramic materials. Finally, electrically insulating oils or fats carried by the medium can penetrate into the pores of the ceramic, interrupting the galvanic connection between the medium and the metal electrode.